Among the most interesting monuments of ancient Eretria is the Iseion, a temple sacred to the goddess Isis and other Egyptian deities. Situated to the south of the town, between the baths and the Lower Gymnasium or the palaistra (wrestling area), it extends behind the small harbour, a detail that correlates the temenos with merchants who had their interests in Eretria. According to excavation and inscription testimonies, the temple was probably built in the fourth century BC and was surrounded by other edifices and auxiliary spaces. The initiation to the cult of Iris and the Egyptian deities occurred during the Hellenistic period by Greek merchants who came to Greece from Egypt after the unification of the then known world by Alexander the Great. Their worship in Eretria has also been attested by inscriptions, of which the most important is set on a limestone block to the left of the prodomos (anteroom) before the cella.

The temple of Isis was initially simple and oriented to the east, with a prodomos that was distyle (two-columned) in antis. The ceremonial clay statue of the goddess stood on a base within the cella. In front of the temple was the altar and nearby a small drain tank. The temple was reconstructed after the destruction of the city by the Romans in 198 BC: it then acquired a larger external prodomos on ameliorated foundations and was surrounded by porticos on three sides (north, south and west). Only the southwest end of the portico was covered by a roof. The columns were later replaced by a parapet. At the centre of the east forecourt was a portal facing the entrance of the sanctuary. Fifteen more edifices and auxiliary spaces lied to the north, considered by the excavators as places of purification. Among them was a courtyard and an andr?n (dining hall for male residents), while one room of the complex had a superb mosaic floor featuring lozenges.

Excavations at the temenos sacred to Isis and other Egyptian deities were conducted in 1917 by the then Ephor of Antiquities for the island of Evia (Euboea), Ι. Papadakis. In recent years, the Archaeological Service of the Ministry of Culture undertook further excavations in the wider area of the temple, which brought to light an additional complex of courtyards and rooms directly related to the sanctuary.